Had this fixed about a month ago, new sensor (genuine part) and labour cost me £150

All fine, then turned the car on this morning and the error light came on again!
Plugged in the diagnostic reader and its the same error code (just one this time); 16519!
Called the garage and after i told him the same error has come up on the problem he had fixed, he responded with a long "so what" silence?
Earliest he can see the car is the 10th August!
He said the part is guaranteed, but they very rarely go wrong?
What could it be? just that the part is a dud or bad workmanship?

Cheers